Leveling floor joists

Doing a full bathroom remodel. I've ripped out about half the subfloor to replace rotted plywood. I'm thinking of ripping the rest out now because the floor is so out of level.

From the outside wall to the interior wall the joists are almost an inch out of level over 5'. The house is obviously sloping inwards.

My plan was to use 2x4s and cut them to correct the slop and glue and screw them to the existing joists to get them back to level running the length of the joist. My real question is how do I make sure that the perpendicular is still level (or just close to level doesn't have to be perfect) so when I put the new subfloor down i don't have heaves and valleys.

Just get everything closer to level so its not so drastically out of level and then use a self leveling compound?

You can pull strings across the floor and bring the new joists up to that level.
Or a laser level.

You may want to look at jacking up an resupporting too. I had a home that had dropped in the middle. I went in the crawl space and saw that the original framers had not used all of the concrete pads under the stairwell. I moved those into place, rented some floor jacks and lifted things up, and resupported them. Doors that hadn't closed right before now worked.

Thanks for the info guys. Since the house is sitting the way it is I think I'm just going to leave it alone as far as trying to jack it up. I'll just tear out the rest of the subfloor and go get some 2x6s. Once this is done the project should hopefully start flowing again.

Well the sistered in 2x6s are almost complete the next question is the transition from the bedroom is an inch and a half of subfloor made up of a 1/2 inch plywood covered with with 3/4 inch particle board with fairly thick carpet on top

To get the the transition height from the bedroom into the bathroom close should I just use 3/4" plywood and then put another 1/2" sheet of plywood on top of that with more gluing and screwing so it doesn't squeak?

If the first layer of ply is installed properly, it's safer to not glue the second layer to it. If you decide to do it, you need a liquid wood glue like Titebond II, a trowel to spread it around so you get full coverage, and lots of screws and be fast about it so you get it spread and the second layer down before it skins over. DO NOT use a construction adhesive or you'll be creating problems as it's nearly impossible to get 100% coverage. Construction adhesive on the top of the joist works because the joist is only 1.5" across, and it's not that hard to get enough force to get intimate contact. You can't do that with construction adhesive spread over the whole sheet. Regardless, when installing the second sheet, do not line up the joints with that below. In fact, run the end 1/4-span past the joist (on 16" OC, the end should be 4" off the joist center). The whole goal of the second sheet it so cover the first layer's end joints on the joist and isolate the minor jacking caused by deflection mid-span...1/4-span is the ideal distance to minimize that. In your case, unless you're planning natural stone (which would require the second layer of ply), you're doing this more for height verses strength. Just screws (NOT into the joists) is sufficient to keep the layers intimately attached, assuming you use enough in the proper spacing. You then have a choice of cbu or a decoupling membrane on top of the ply before you tile. The decoupling membranes available are thinner than cbu for the most part (DitraXL is an exception and is used rather than Ditra when you need extra height). Membranes, in my experience, are a lot easier to cut, carry, and install verses cbu - just thinset to install, and it cuts with a sharp knife or scissors. You can carry all you need for a big room easily on your shoulder, and it works better! Also, if you seal the seams, it's waterproof, should that be of interest.